1. Field of the Invention
The present invention relates to a magnetostrictive transducer for generating a torsional wave in a subject using magnetostriction effect, and more particularly, to an apparatus for both generating a torsional wave in a subject such as a shaft or a beam having a predetermined cross section and measuring the torsional wave propagating in the subject.
2. Description of the Related Art
Magnetostriction refers to the mechanical strain that occurs in ferromagnetic materials when they are subjected to a magnetic field. It is also referred to as the Joule effect. Conversely, an inverse magnetostrictive effect refers to the change in magnetization of ferromagnetic materials when subject to mechanical stress, which is also referred to as the Villari effect.
Since transducers that exploit magnetostriction can measure a strain in a subject without being in contact with the subject, they have been applied to various fields that cannot use contact sensors. The use of magnetostriction makes it possible not only to generate elastic ultrasonic waves in a non-contact manner but also to generate ultrasonic waves having larger amplitude than that of ultrasonic waves generated by conventional techniques that use the piezoelectric effect. Ultrasonic waves that can be generated from a waveguide body, e.g., a rod or a piping member, include longitudinal, transverse, and torsional waves.
In the first mode of the torsional waves, little dispersion occurs. Dispersion indicates a velocity variation depending on frequency. Thus, the first mode of the torsional wave can be effective in evaluating the structural characteristics of a rod or a piping member.
In general, ferromagnets include substances such as iron, nickel, and cobalt. Among these ferromagnets, nickel is used in an embodiment of the invention as it is a soft magnetic material characterized by having a sharply rising magnetization curve and small hysteresis and returning to its original shape after removal of a magnetic field.
FIG. 1 shows group velocity curves with respect to frequencies in a torsional wave propagating in a rod member. The rod member is an aluminum pipe having a thickness (t) of 1 mm, an outer diameter (d0) of 25 mm, a density (ρ) of 2800 kg/m3, and an elastic coefficient (E) of 73 GPa.
As shown in FIG. 1, dispersion hardly ever occurs in the first mode among the group velocity curves. Thus, for example, in remote detection, if a non-destructive testing is carried out using a torsional wave, it is be very helpful in detecting defects of a pipe or the like as it maintains its form upon reflection.
As shown in FIG. 2, a conventional magnetostrictive transducer includes a thin nickel strip 1 that is wound around a subject 2, an insulator 4 installed around the nickel strip 1, and a coil 3 wound around the insulator 4.
When a defect of the subject 2 is scanned using the conventional magnetostrictive transducer, the nickel strip 1 is wound around and attached to the cylindrical surface of a pipe, and is magnetized in the circumferential direction of the strip 1 using a permanent magnet (not shown). An elastic wave is generated by applying a magnetic field to the coil 3 around the magnetized nickel strip 1 and this generated elastic wave is measured. When the magnetic field is applied to the nickel strip 1, a torsional wave is generated in the subject 2. The generated torsional wave propagates along the subject 2 and is reflected back from an end of the rod member on an area having a structural defect. Then, the magnetic field of the nickel strip 1 is changed by the reflected torsional wave.
FIGS. 3A and 3B are graphs showing signals measured using a elastic wave (particularly, a torsional wave) generated by the conventional magnetostrictive transducer shown in FIG. 2. An input current to the coil 3 is 3 ampere in FIG. 3A and is 6 ampere in FIG. 3B.
As shown in FIGS. 3A and 3B, since a waveform generated by an input current in the conventional magnetostrictive transducer is very sensitive to magnetic intensity, it is difficult to generate a torsional wave having a large amplitude. As the magnetic intensity of an alternating magnetic field generated in the coil increases, the magnetic intensity of a magnetic field caused by the latent magnetization of the nickel decreases. Thus, as shown in FIG. 3B, a longitudinal wave other than the torsional wave is unintentionally generated resulting in a complex signal and making it difficult to distinguish the torsional wave from other waves.
In other words, if amount of pre-magnetization of the nickel strip 1 in the circumferential direction of the subject 2 and a magnetic intensity along the axis of the coil 3 are not appropriate, the conventional magnetostrictive transducer may generate waves other the a torsional wave. Also, since the nickel strip 1 is a soft magnetic material, it should be pre-magnetized again to be used for an extended period of time.
To overcome such a problem, the inventor of the present invention has proposed a new method as published in a paper entitled “Generation of Induced Torsional Wave and Detection of Defect of Pipe Using Magnetostrictive Transducer” in Transactions of the Korean Society for Noise and Vibration Engineering (Vol. 14, Second Edition, pp. 144-149, 2004; Yoon Young Kim et al.).
According to the method suggested in the above-stated paper, as shown in FIG. 4, a rectangular nickel patch 9 is attached to a subject 2 that is to be scanned at an angle of 45° with respect to the axis direction of the subject 2. The elastic wave generated by attaching the nickel patch 9 in the above-described manner is a torsional wave, unlike in conventional magnetostriction. The generated torsional wave propagates along the subject 2 and is reflected back from the other end or a defective area of the subject 2.
When a current is input to a solenoid coil 3 wound around the nickel patch 9, a magnetic flux is also created at an angle along the nickel patch 9. At the same time, the nickel patch 9 generates a torsional wave mainly by changing the direction of its magnetic field based on magnetostriction. Since a transducer 10 including the nickel patch 9 is able to generate a torsional wave without pre-magnetization or a bias magnetic field, it can be very efficient in generating a torsional wave.
FIGS. 5A and 5B are graphs showing results of experiments carried out using a rectangular patch 9 of 25 mm×3 mm according to the method suggested in the aforementioned paper. In detail, FIG. 5A shows an experimental result when a current of 4.75 ampere is applied to the coil 3 shown in FIG. 4, and FIG. 5B shows an experimental result when a current of 8.44 ampere is applied to the coil 3 shown in FIG. 4. From comparison between the results shown in FIGS. 5A and 5B with those shown in FIGS. 3A and 3B, it can be seen that no other waves than a torsional wave is generated by the method suggested in the aforementioned paper even when an applied voltage is increased. For example, when using the torsional wave to scan for defects, it is possible to accurately detect where the defect is, as confirmed from the fact that there is no distortion in the generated pulse.
While the aforementioned method suggested in the paper has proven to be an efficient method, there is a still need for a transducer capable of generating an output having both a considerable amplitude at a small input signal and a better signal-to-noise ratio.